I've commercially fished, and slaughter animals every year, and have (unfortunately) dispatched more fish and other creatures than I'd like to remember. Far more than most people, I'm certain.
There are two basic ways to kill a fish humanely- slice them with a razor-sharp knife to bleed them out or crush their brain in a single blow.
The knife technique is probably the safest and least traumatic to the fish, if done right, and it is the method I use the most. A quick slice across the throat, and let the fish go into the livewell. It will not struggle or flail about, it will simply continue swimming until it passes out. An easy way to go. One large benefit is that once a fish is bled in this manner, the meat will be whiter and "cleaner" tasting, and it will not spoil as quickly. And the livewell pumps out all the blood, so there is no mess to clean up.
The second technique is to bludgeon the fish. The little baseball bats look nice, but are NOT very efficient, especially on smaller fish. They are too wide. Have seen guys beating and beating on fish with those stupid little baseball bats and the fish keep waking up because all they do is knock them out. A hammer can work, but they are too short, and only effective if the person makes a perfect shot. A terrible tool for this work.
The real fish dispatchers are 18-24" long, but only about 3/4" in diameter and are relatively heavy. Kind of like the "asp" the military uses for security work, they reach a high tip speed, and then all of that speed and weight and force is concentrated into a very narrow point of impact. THAT is what crushes the skull in a smooth, effortless motion. These are MUCH easier to use than a hammer or bat-type instrument.
Mine is a stainless steel commercial model, made specifically for the task, but you can make your own from an 18"-24" piece of 3/8" galvanized water pipe or 1/2" rebar. You can make it with or without caps or tape grips.
And before any of you question what I say, try it. Not only do I think you will be in agreement, it would not surprise me if you weren't somewhat frightened from the devastating damage it inflicts, especially if you swing it like you must with the lesser tools. Do not play around with these things. They are NOT toys, and minors should not be allowed to handle them.
One smooth swing is all that is needed, and the fish will be dead instantly. If you are after 100# cobia, you might want to up it to 1/2" pipe.
That being said, I personally use a stroke of the fillet knife, except for cobia. Super-clean table fare is my goal, as well as killing humanely.
-TH